This invention relates to high intensity discharge lamps and in particular to reflective coating patterns for the discharge tube.
High intensity arc discharge lamps of the kind discussed herein are typically found in street lighting and parking lot lighting applications and other applications requiring a high intensity, efficient light source. These lamps conventionally operate by passing electrical current through an ionizable vapor typically contained in a transparent quartz discharge tube. At either end of the quartz tube there is an electrode for carrying the ionizing electric current. The operation of these lamps is similar to the operation of conventional fluorescent lamps except that the electrodes herein need not be heated filaments as in fluorescent lamps and a phosphor coating is not required for the production of light output; in these lamps, the excitation of the ionizable vapor itself is responsible for the emission of visible wavelength photons. The discharge tube itself operates at a relatively high temperature and accordingly it is typically contained in an exterior envelope not only for safety but also for the purpose of maintaining the discharge tube at a sufficient temperature so that the desired vapor pressure of the ionizable medium within the discharge tube is maintained. The ionizable medium typically comprises a mercury-metal halide mixture but other ionizable media may also be employed, the exact mixture often being dependent upon the desired color spectrum of the emitted light. To conserve heat in the discharge tube, a white or metallic reflective coating is employed to conserve heat energy in the tube to maintain the desired vapor pressure. The coating typically extends along the arc tube wall beyond the tips of the electrodes which are typically disposed at opposed ends of the tube. Accordingly, considerable useful radiation is partially trapped within the tube by multiple internal reflections which results in a reduction in efficacy of as much as approximately 10 percent as compared to other methods of maintaining the vapor pressure.
Vertical operation of such high intensity discharge lamps typically poses few problems with respect to the arc location within the envelope. However, during horizontal operation there is a tendency for the arc discharge to bow upwards often contacting the top wall of the arc tube due to convection and buoyancy of the less dense, hot plasma in the region dissipating the greatest amount of energy. This effect results in a loss of efficiency due to cooling of the plasma by conduction through the top wall of the arc tube and also results in shortened life and even possibly catastrophic lamp failure due to overheating of the top wall of the quartz envelope which typically operates at a temperature of approximately 900.degree. C. Two basic configurations have been proposed to reduce this problem. In the first configuration, the discharge tube itself is arched so as to conform to the natural upward curvature of the plasma arc. Such an arched discharge tube is described, for example in U.S. Pat. No. 4,142,122 issued to Koza. A second configuration which reduces this problem by arc bowing is to place a structure producing a magnetic field in the vicinity of the lamp so as to confine the arc discharge and remove it from the vicinity of the discharge tube wall. Such a configuration is described, for example, in U.S. Pat. No. 2,027,383 issued Jan. 14, 1936 to C. E. Kenty and also in application Ser. No. 945,559, filed Sept. 25, 1978, the latter being incorporated herein by reference. These configurations, which are particularly useful for horizontally operated high intensity discharge lamps, have a unique feature in common in that they both generally require the lamp to be oriented in a particular direction for proper operation. For example, in the case that the discharge tube is arched, the lamp is generally required to be positioned so that the peak of the arch is also the highest point gravitationally. This generally requires a definite orientation for the lamp and likewise tends to require the use of a bayonent-type base, rather than a screw-in base for the lamp.